arch/tile: provide PT_FLAGS_COMPAT value in pt_regs

This flag is set for ptrace GETREGS or PEEKUSER for processes
that are COMPAT, i.e. 32-bit.  This allows things like strace
to easily discover what personality to use, for example.

Acked-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Chris Metcalf <cmetcalf@tilera.com>